ABSTRACT: We measured the isotopic composition and accumulation of particulate organic matter (POM) and the uptake of carbon (C) and nitrogen (N) in an early bloom of the most productive recurring polynya of the Arctic Ocean. The estimated compensation irradiance at the onset of the bloom was similar to the average for the North Atlantic Ocean, implying that shallow mixing was of critical importance for the blooms early initiation. Planktonic POM had a much lower δ13C than ice POM, suggesting that ice-algae contributed little to the pelagic biomass. The overall isotopic fractionation of pelagic N during bloom development was consistent with in situ diatom growth under saturating irradiance and limiting NO3-. Soon after the ice cleared, rapid physiological changes induced an order of magnitude increase in the C and NO3- uptake capacity of diatoms, leading to very high f ratios (NO3- uptake : total N uptake). Most of the NO3- taken up appeared in the POM, so that little net release of reduced N occurred during the period of active growth. Given the tight coupling between photosynthesis and NO3- uptake under N limitation, the magnitude of primary production in the Arctic Ocean is expected to respond to changes in N supply.

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